Which one of PCl₄ and PCl₅ is not likely to exist and why?

When phosphorus accepts 3 electrons from 3 chlorine atoms it completes its octet, so it is unable to take one more electron from one more Cl⁻ so the formation of PCl₄ is difficult. As we move down the group, the stability of +5 oxidation state decreases and the stability of +3 oxidation state increases. Chlorine belongs to the third period. Hence, PCl₅ compound is more likely to exist.